The present invention relates to radio communication technologies and more particularly, to a mobile radio communication technique for performing radio communications in which a radio apparatus performs radio communications by using a multi-antenna composed of a plurality of antennas.
In the field of mobile radio communication, a MIMO (Multi-Input Multi-output) system has been widespread in which both radio apparatuses adapted to perform transmission and reception of radio signals have each a plurality of antennas. In the MIMO system, transmission information is transmitted from the plural antennas and is then received by the plural antennas by way of a plurality of propagation paths (a multi-path). In the MIMO system, signals are multiplexed spatially and so, a plurality of pieces of different information are transmitted simultaneously at the same frequency, received by the plural antennas and then separated, so that highly efficient signal transmission can be achieved.
The signal reception processing in the MIMO system can be sorted into a process of decomposition of the MIMO for separating a plurality of coded signals from the multiplexed signal inputted to the reception antenna and a decoding process for returning each decomposed coded signal to information. As is known in the art, a MIMO channel reception process based on MLD (Maximum Likelihood Detection) and adapted to simultaneously perform the decomposition process and the decoding process can give optimum solutions. In the MLD, however, the quantity of operations increases exponentially in accordance with the number of states and the number of antennas and so, in present situation where resources of signal processing are limited, the MLD is difficult to be put into practice. The number of states referred to herein indicates the number of choices (varieties) capable of being taken. For a signal of 16 QAM (Quadrature Amplitude Modulation), for example, there are 16 kinds of choices. In the MLD, a method is taken according to which all choices are considered and a choice of the highest possibility of them is selected. Therefore, as the number of choices (states) increases, the quantity of operation increases correspondingly.
For the purpose of suppressing the operation quantity to realistic one, a MMSE (Minimum Mean Square Error) method has been known according to which the decoding process and the decomposition process of MIMO are carried out stepwise. The MMSE is superior to the MLD from the standpoint of smaller load imposed upon operation process but in a fading condition, the signal quality (SINR: Signal to Interference and Noise power Ratio) for obtaining an equivalent packet error rate is degraded by about 3 to 10 dB. Comparison of the MLD to the MMSE is explained in NTT DoCoMo technical journal Vol. 14, No. 1 entitled multi-antenna radio transmission technique (Non-patent Literature).
In the MIMO, a plurality of propagation paths is established virtually on space by using a plurality of antennas. One of the propagation paths established on space axis is called a stream. A code word is a name concerning a unit of information carried on the stream. One code word is generated by a single encoder. Two code words are generated by means of two encoders. For example, one code word generated with the encoder can be so decomposed as to be transmitted on two streams and this is called single code word transmission. A multi-code word transmission is also possible in which a plurality of code words generated by a plurality of encoders are transmitted from a plurality of antennas. In the multi-code word transmission, a plurality of users can each transmit code words with the help of a single encoder. This is called MU-MIMO (Multi-user MIMO). The code word has relation to the encoder (propagation path encoder) and error correction is applied in unit of code word. Accordingly, when two code words, for example, are transmitted, there is a possibility that one code can be corrected for error owing to the effect of error correction and can be received correctly but the other code word will fail to be corrected for error and cannot be received correctly.